Thunderbird Portable seems to be Windows-only. I use GNU/Linux, so that wouldn't help me. But I'm sure there are other options for Linux.
Even though I use IMAP, I didn't lose any mail. Luckily, after the last outage, I saw the posts here about lost mail, so I avoided using my local client (Thunderbird). If I hadn't seen the posts, I suppose Thunderbird would have synched with the server. But still, I do daily backups that include the Thunderbird folder, so at worst I wouldn't have lost more than 24 hours of mail.